High output voltage swing class AB operational amplifier output stage

Abstract

An output stage (10) is provided having transistors of the same polarity type, which function to amplify the input signal at a wide range of frequencies, with low power consumption and low crossover distortion. By biasing the output transistors to remain on during both the positive and negative voltage swing of the input signal, low power consumption as well as low output crossover distortion is achieved. A high efficiency, low crossover distortion, current amplifier circuit for amplifying an input signal (VIN) in accordance with this present invention includes an output driver (12), a current source (IS1) and a translinear loop circuit (14). The output driver (12) includes a sourcing circuit (Q6). The current source (IS1), connected to the output driver (12), provides bias current to the sourcing circuit. The translinear loop circuit (14), connected to the output driver (12), receives the input signal (VIN). The translinear loop (14) includes a sinking circuit (Q7), such that the translinear loop circuit (14) is responsive to the a sinking current of the sinking circuit and operable to provide a bias current signal proportional to the sinking current.

Description

This application claims priority under 35 USC § 119(e)(1) of provisional application No. 60/142,494, filed Jul. 6, 1999.

FIELD OF THE INVENTION

This invention relates generally to the field of operational amplifiers; and, in particular, to an amplifier having minimal zero crossover distortion and quiescent current requirements.

BACKGROUND OF THE INVENTION

Well known in monolithic integrated circuit design, the design of bias circuitry internal to the chip is essential since it determines the internal voltage and current levels over all operating conditions of the integrated circuit as well as over all manufacturing process variations. The industry trend for electronic systems encompassing operational amplifiers is evolving toward lower operating voltages supplied from battery sources. Thus, amplifiers are used in applications requiring low voltage single supply operations in addition to traditionally desired operational amplifier properties such as high input impedance, low input offset voltage, low noise, high bandwidth, high speed and sufficient output drive capabilities.

The operational amplifier consists of at least two stages: an input amplifier stage and an output stage. The input amplifier stage has the task of deriving the difference between the two inputs. The primary purpose of the output stage is voltage amplification. The output stage optionally includes a current boosting scheme which increases the amplifier's load capacity. Conventionally, amplifier output stages have used techniques involving combinations of transistors including npn, pnp and metal oxide semiconductor field effect transistors to satisfy many performance specifications, such as low crossover distortion, high gain factor, large output voltage swings including rail to rail performance, excellent phase and gain margins, low output impedance and symmetrical source and sink capabilities. A well-designed output stage should achieve these performance specifications while consuming low quiescent power and not limiting the frequency response of the amplifier.

During operation, an output stage consumes current from a power supply. A portion of this current, known as the quiescent current, is used to bias the internal circuitry of z; the output stage. The quiescent current is the current required to bias all transistors on when the input is shorted to ground. The purpose of the quiescent current is to provide sufficient base current for the amplifier's output transistors and current to maintain the amplifier's critical circuitry “on.” Low quiescent current is desirable because it reduces power consumption when the amplifier is operating at a light load, or with no load at all. The active components of an all NPN Class AB amplifier include two NPN transistor, wherein the emitter of the first transistor is connected to the collector of the second transistor. The first NPN transistor amplifies the input signal during the positive swing of the signal, and the second NPN transistor amplifies the input signal in its negative swing. The active components may include diodes, resistors and transistors, with increased bias current for the two transistors to reduce crossover distortion.

Traditional Class AB output stages of an operational amplifier are capable of driving a specified minimum impedance load while possessing a low quiescent current. Traditional Class AB output stage “sink” or “source” a significantly larger current in the presence of low impedance loads. Without a quiescent current large enough to bias all transistors critical to the output stage, these transistors will be “cut off” in an effort to drive a low impedance load. Turning this critical circuitry on again, ready for the next amplifier output transition increases crossover distortion.

Conventional AB class output stages drive a minimum impedance load having a given crossover distortion. Where lower impedance loads exist, higher quiescent current is required within the output stage. Going beyond output stage's specified limits of load results in increasingly considerable crossover distortion.

In conclusion, there are existing designs that minimize crossover distortion of the output stages of an amplifier by allowing the active devices to conduct a small amount of current at the crossover point. Some arrangements use both NPN and PNP output transistors which requires more space and operates slower than an all NPN output transistor arrangement. An arrangement utilizing all PNP transistors also requires more space and operates slower than an all NPN output transistor arrangement. These techniques are limited by the quiescent current; hence, a need exists for an output stage that significantly minimizes crossover distortion using an all NPN output transistor arrangement with no quiescent current requirements.

SUMMARY OF THE INVENTION

The present invention relates to an output stage, having transistors of the same polarity type, which function to amplify the input signal at a wide range of frequencies, with low power consumption and low crossover distortion. By biasing the output transistors to remain on during both the positive and negative voltage swing of the input signal, low power consumption as well as low output crossover distortion is achieved. A high efficiency, low crossover distortion, current amplifier circuit for amplifying an input signal in accordance with this present invention includes an output driver, a transistor, a resistor, a current source and a translinear loop circuit. The output driver includes a sourcing circuit. The current source, connected to the output driver, provides bias current to the sourcing circuit. The value of the resistor determines whether the transistor remains on at all times and should not be set such that reverse bias occurs in the sourcing circuit. This resistor enhances the ratio of transistor emitter area between the transistor and a second transistor of the sourcing circuit. The translinear loop circuit, connected to the output driver, receives the input signal. The translinear loop includes a sinking circuit, such that the translinear loop circuit is responsive to the sinking current signal and operable to provide a bias current signal proportional to the sinking current.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which like reference numbers indicate like features and wherein:

FIG. 1 is a schematic of an embodiment of an output stage in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an output stage 10 in accordance with the present invention. This high efficiency, low crossover distortion, output stage 10 includes a current source IS1, an output driver 12, a biasing transistor Q3, a resistor R2 and a translinear loop 14. The translinear loop 14 includes transistors Q1 Q2, Q4, Q5, and Q7, a resistor R1, and a second current source IS2, Current source IS2 not only biases the translinear loop 14 but also increases the speed of this Darlington pair. The output driver 12 comprises sourcing circuit which includes a sixth transistor Q6. The collector of the first transistor Q1 is connected to a first power supply rail having a positive biasing potential +VCC. An input node VIN is connected to the base of the first transistor Q1. The first resistor R1 is connected to the emitter of the first transistor Q1. The collector and the base of the second transistor Q2 are connected to resistor R1. R1 is optional since it serves primarily to keep the quiescent current low. The emitter of the second transistor Q2 is connected to a second power supply rail −VCC. The first current source IS1 is connected to the first power supply rail +VCC. The collector and base of the third transistor Q3 are connected to the first current source IS1, The collector of the fourth transistor Q4 is connected to the emitter of the third transistor Q3. The base of the fourth transistor Q4 is connected to the base of the second transistor Q2. The emitter of the fourth transistor Q4 is connected to the second power supply rail −VCC. The collector of the fifth transistor Q5 is coupled to the first power supply rail +VCC, while the base of the fifth transistor Q5 is connected to the base of the first transistor Q1. The second current source IS2 is connected between the emitter of the fifth transistor Q5 and the second power supply rail −VCC. The base of the seventh transistor Q7 is connected to tile second current source IS2 and the emitter of the seventh transistor Q7 is connected to the second power supply rail −VCC. Current source IS2 biases transistor Q5 and helps to bias the translinear loop. It helps to keep transistor Q5 on when the circuit 10 is in sinking mode. Transistors Q5 and Q7 form a Darlington pair. The collector of the sixth transistor Q6 is connected to the first power supply rail +VCC. The base of the sixth transistor Q6 is connected to the collector of the third transistor Q3. The emitter of the sixth transistor Q6 is connected to an output node OUT. The second resistor R2 is connected between the emitter of the third transistor Q3 and the output node OUT. The collector of the seventh transistor Q7 is connected to the output node OUT.

Transistors Q6 and Q7 are the output transistors of the output stage 10. When the AC input signal applied to input node VIN is in the positive range, or is above the crossover point, the circuit is said to be “sinking current” through the output node OUT. Transistor Q1 is on, drawing current I1 from the first power supply rail +VCC. Current I1 of transistor Q1 flows through resistor R1 and transistor Q2. Since transistors Q2 and Q4 form a current mirror, current I2 is equivalent to I1AQ2/AQ4, where AQ1 and AQ2 are the emitter areas of transistors Q1 and Q2. Current I2 flowing through transistor Q4 comprises the current that is generated by the current source IS1 and the current that flows across resistor R2 from the output node OUT. The current I3 of transistor Q5 is equal to the current from the second current source IS2 added to the current Ibase applied to the base of transistor Q7, such that transistor Q7 is biased by the translinear loop formed by transistors Q1, Q2, Q4, Q5 and Q7.

The voltage developed across R2 should not be large enough to reverse bias the base emitter junction of transistor Q6. A small amount of current generated by the current source IS1 is applied to the base of sourcing transistor Q6. Thus, the output transistor Q6 remains on when the output transistor Q7 is sinking current.

More particularly, when AC input signal applied to input node VIN has a positive voltage swing, the voltage applied to the base of transistors Q1 and Q5 is positive and thus, the emitter voltage of transistor Q1 and Q5 is positive as well. The positive voltage applied to the base of both transistors Q1 and Q5 increases the current I1 and I3 that flows through each transistor Q1 and Q5, respectively. Current I1, flowing through resistor R1 and transistor Q2, increases. Since current I2 is the mirror of current I1, current I2 increases relative to the increase in current I1. The increase in current I2 is drawn from the current supplied by the current source IS1 and the current through the output node OUT across resistor R2. As a result, less current is applied to the base of transistor Q6. Transistor Q6, however, is left with enough current to keep it on; yet, it does not source any current through the output node OUT.

Transistor Q5 is more sensitive to changes in the input voltage applied at input node VIN than transistor Q1. The emitter voltage of transistor Q5 is applied to the base of transistor Q7. Since the input voltage VIN increases the base emitter voltage of transistor Q5 with respect to −VCC, a positive voltage is applied to the base of transistor Q7. As a result, the collector voltage of Q7 is negative—the output stage 10 is thus sinking current. Due to the increase in current I3 through transistor Q5, more current is applied to the base of transistor Q7. Accordingly, transistor Q7 sinks more current.

Transistors Q1, Q2, Q4, Q5, and Q7 form a translinear loop. The Kirchoff's voltage equation provides that:

VbeQ1+VbeQ4=VbeQ5+VbeQ7, assuming R1=0  (1)

Vtln(I1/AQ1IS)+Vtln(I2/AQ4IS)=Vtln (I3/AQ5IS)+Vtln (I4/AQ7IS)  (2)

I4=Isink=(I1I2AQ5AQ7)/(AQ1AQ4I3)  (3)

where VbeQ1, VbeQ4, VbeQ5, and VbeQ7 represent the base-emitter voltages of transistors Q1, Q4, Q5, and Q7, respectively. Areas AQ1, AQ4, AQ5, and AQ7 represent the emitter area of each respective transistor. Current IS is the inverse saturation current. Thus, the sinking current Isink is proportional to the boosting current I2 of transistors Q3 and Q4. Since transistors Q2 and Q4 form a current mirror, current I2 is equivalent to I1AQ2/AQ4, where AQ1 and AQ2 are the emitter areas of transistors Q1 and Q2. Thus, the sinking current Isink is proportional to the square of the current provided by the input signal or the current source IS1.

The sinking current Isink becomes very large when I1 and I2 have unlimited supply current. The power supply generates current I1. Current I2 comprises current from the current source IS1 and the current being sourced from the output node OUT through resistor R2. During sinking conditions, the only current drive limitation is determined by the available base current to transistor Q5 and Q7. More particularly, this current is represented by Isink/hfeQ5*hfeQ7; since:

I4=IB7 hfeQ7

IC5=IB5 hfeQ5

Isink=IB5 hfeQ5 hfeQ7

IB5=Isink/hfeQ5 hfeQ7

Thus, there is no significant limiting factor for the current in the output stage sinking current from a given load.

The circuit significantly depreciates crossover distortion in the output signal, by keeping transistors Q6 and Q7 on at all times. As the input signal applied to input node VIN approaches the crossover point during either a falling transition, the amount of current I2 drawn from the current source IS1 is reduced. When the input signal applied to input node VIN reaches the crossover point, the amount of current drawn by transistor Q3 is minimal yet the transistors Q3 and Q4 remain biased on. At this stage, virtually no load current is being supplied by the circuit.

When the input voltage applied to input node VIN is at the crosspoint or is negligible, the current I3 through transistor Q5 will be approximately equal to current from current source IS2. Current I1 flows through transistors Q1 and Q2 and resistor R1. The current source IS1 ‘quiescently’ biases transistors Q3 and Q6. Current I4 is not large enough to sink current from the output load. The base-emitter voltage of transistor Q7 influences current I3 which flows through transistor Q5. The base-emitter voltage of transistor Q5 is set by the current I3. Since the bases of transistors Q1 and Q5 are coupled together the base-emitter voltage of transistor Q1 added to the base-emitter voltage of Q2 is the same as the base-emitter voltage of transistor Q5 added to the base-emitter voltage of transistor Q7. Current I1 is determined by the sum of base-emitter voltages of transistors Q1 and Q2. Since transistors Q2 and Q4 form a current mirror, current I2 is equivalent to I1AQ2/AQ4. Resistor R2 is set such that no current comes through the output node OUT to be added to current I2. Thus, current I2 equals the current from current source IS1 solely. In summary, at the quiescent point, all transistors are conducting current and are set by the current source IS1.

When the input voltage applied to input node VIN swings in the negative direction, the voltage across transistors Q1 and Q5 is negative. The negative voltage is applied to the base of transistors Q1, Q2, and Q4. The collector voltage of transistor Q4, however, is positive. This positive voltage is applied to the emitter of transistor Q3. As a result, this positive voltage is applied the base and collector of transistor Q3 and to the base of transistor Q6. At this point, the amount of current I2 drawn from the current source IS1 decreases. Yet, the amount of current drawn by transistor Q3 is maximum. A large amount of current from current source IS1 is applied to the base of transistor Q6. At this point, transistor Q6 conducts current. The circuit is said to be “sourcing current” through the output node OUT to a load connected to the output node OUT.

The dynamic biasing which boosts transistor Q6 during sourcing mode of the output stage depends upon the amount of current through transistor Q3 and resistor R2. If the resistance of resistor R2 is small enough to make the voltage across it to be zero, then the ratio of the emitter area of transistors Q3 to Q6 biases the output transistor Q6 during the sourcing mode of the output stage. It is the purpose of resistor R2 to enhance the ratio of transistor's Q3 and Q6 emitter area during sourcing mode to provide a larger ratio, such that a small current through transistor Q3 results in sourcing a large current through transistor Q6. Using Kirchoff's voltage equation rules, the base-emitter voltage of transistor Q6 is equal to the voltage across resistor R2 added to the base-emitter voltage of transistor Q3.

Since current I2 decreases, current I3 decreases and, as a result, the base-emitter voltage of transistor Q7 decreases. Transistor Q7, however, remains on. Transistor Q7 is more likely to remain “on” during sourcing since the translinear loop will tend to keep all transistors operating as long as a small amount of current flows through transistor Q4. Since the current path to transistor Q7 is through transistors Q3 and Q5, as well as resistor R2, transistor Q7 will remain “on” as long as the voltage drop across R2 is maintained under couple hundred millivolts. In addition, the value of resistor R2 determines whether Q3 remains “on.”

Those skilled in the art to which the invention relates will appreciate that various substitutions, modifications and additions can be made to the described embodiments, without departing from the spirit and scope of the invention as defined by the claims.

Claims

1. A high efficiency, low crossover distortion, current amplifier circuit having an input and output node for amplifying an input signal comprising:

a first current source for providing bias current to a sourcing circuit coupled to a first power supply rail;

an output driver coupled to the first current source and the output node, the output driver including a sourcing circuit;

a first transistor having a collector, base and emitter, the base and collector coupled to the current source;

a resistor coupled between the emitter of the first transistor and the output node; and

a translinear loop circuit coupled to the input node, the first transistor, and the output driver, the translinear loop circuit comprising,

a sinking circuit having a second transistor including a collector, base and emitter, the translinear loop circuit responsive to a sinking current of the sinking circuit and operable to provide a bias current signal proportional to the sinking current,

a third transistor having a collector, base and emitter, the collector coupled to the first power supply rail, the base coupled to the input node,

a second resistor coupled to the emitter of the third transistor,

a fourth transistor having a collector, base and emitter, the collector and base coupled to the second resistor, the emitter coupled to a second power supply rail,

a fifth transistor having a collector, base and emitter, the base coupled to the base of the fourth transistor, the emitter coupled to the second power supply rail, the collector coupled to the emitter of the first transistor,

a sixth transistor having a collector, base and emitter, the collector coupled to the first power supply rail, the base coupled to the input node,

a second current source coupled between the emitter of the sixth transistor and the second power supply rail, the second current source coupled to the sourcing circuit, and

wherein the collector of the second transistor of the sinking circuit is coupled to the output node, the base of the second transistor of the sinking circuit is coupled to the second current source, and the emitter of the second transistor of the sinking circuit is coupled to the second power supply rail.

2. The high efficiency, low crossover distortion, current amplifier circuit as claimed in claim 1, wherein the sourcing circuit includes a seventh transistor having a collector, base and emitter, the collector coupled to the first power supply rail, the base coupled to the first current source, and the emitter coupled to the output node.

3. A high efficiency, low crossover distortion, current amplifier circuit having an input and output node comprising:

a first transistor having a base, an emitter and a collector, the collector coupled to a first power supply rail, the base coupled to the input node;

a first resistor coupled to said emitter of said first transistor;

a second transistor having a base, an emitter and a collector, the collector and the base of the second transistor coupled to the resistor, said emitter of the second transistor coupled to a circuit a second power supply rail;

a first current source coupled to the first power supply rail;

a third transistor having a base, an emitter and a collector, the collector and the base of the third transistor coupled to the first current source;

a fourth transistor having a base, an emitter and a collector, the collector coupled to the emitter of the third transistor, the base coupled the base of the second transistor, the emitter coupled to the second power supply rail;

a fifth transistor having a base, an emitter and a collector, the collector coupled to the first power supply rail, the base coupled the input node;

a second current source coupled between the emitter of the fifth transistor and the second power supply rail;

a sixth transistor having a base, an emitter and a collector, the collector coupled to the first power supply rail, the base coupled the first current source, the emitter coupled to the output node;

a second resistor coupled between the emitter of the third transistor and the output node; and

a seventh transistor having a base, an emitter and a collector, the collector coupled to the output node, the base coupled to the second current source, the emitter coupled to the second power supply rail.

4. The current amplifier circuit as claimed in claim 3 wherein the first, second, third, fourth, fifth, sixth and seventh transistors have the same polarity type.

5. The current amplifier circuit as claimed in claim 3 wherein the first, second, third, fourth, fifth, sixth and seventh transistors are NPN transistors.

6. The current amplifier circuit as claimed in claim 3 wherein the first, second, third, fourth, fifth, sixth and seventh transistors are PNP transistors.